The present invention relates to an apparatus for measuring the wear of a doctor blade, which apparatus includes measuring members placed around a used doctor blade for measuring at least one dimension of the doctor blade. The invention also relates to a method in measuring the wear of a doctor blade and in controlling a paper machine.
Doctor blades are generally designed in such a way that they wear in use. This allows the doctor blades to remain sharp and gives the best possible doctoring result. However, the rate of wear of doctor blades varies in individual cases. The rate of wear is influenced by, for example, the blade materials used, the surface being doctored, and the material being scraped off the surface. Particularly in paper machines, long doctor blades are used, in which the wear often varies in the cross-machine direction, i.e. in the longitudinal direction of the doctor blade. The wear also varies from blade to blade between different doctor blades. Usually, experience can be used to provide a sufficiently adequate estimate of the wear. Wear can also be monitored, for example, by measuring the change in angle of the doctor-blade holder. However, the blade holder is often so stiff that a change in the angle of the blade due to local wear in the doctor blade does not show as deformation of the blade holder. This means that it is impossible to determine the wear of the doctor blade from the deformation of the blade holder, at least in connection with so-called rigid blade holders. In other ways too, measuring the change of angle to determine the wear in the doctor blade is an imprecise method, even though online information is then advantageously obtained.
In practice, the actual wear can be measured from a doctor blade, only after it has been removed from its operating location (offline information). It is then possible to determine the wear either by eye, or, for example, by using a vernier gauge to measure different points on the doctor blade. Both ways are imprecise and the measurement in particular is labourious. Measuring doctor blades that can be more than ten-metres long is slow and it is difficult to record the measurement results obtained. In addition, special blade-changing apparatuses, employing bands containing several doctor blades, are nowadays used to accelerate doctor-blade changes. The band is stored on a reel and is moved mechanically. However, it is impossible to measure a doctor blade on a reel. In addition, a single band can last in use for several weeks. It is then impossible to correct sudden disturbances in wear, even though the doctor blade would be measured after being removed from the reel.
The invention is intended to create a new type of apparatus for measuring wear of a doctor blade, by means of which wear can be measured rapidly and accurately. The invention is also intended to create a new type of method for measuring wear of a doctor blade, by means of which the actual wear and its shape can be determined. Further, the invention is intended to create a new type of method for controlling a paper machine, by means of which the control is more rapid and precise than previously.
Accordingly, an apparatus according to the present invention for measuring wear of a doctor blade, which apparatus includes measuring members placed around the used doctor blade for measuring at least one dimension of the doctor blade, is characterized in that the measuring members are arranged to permit longitudinal movement of the doctor blade between the doctor blade and the measuring members and that to the measuring members are connected measurement sensors for obtaining a continuous measurement result when the doctor blade or the apparatus is moved.
Correspondingly, a method according to the invention for measuring wear of a doctor blade, in which method at least one dimension is measured at different points on a worn doctor blade, is characterized in that the dimension is measured essentially continuously while the doctor blade moves and at the same time the longitudinal movement of the doctor blade is measured, so that the longitudinal wear profile of the doctor blade is determined from the results obtained.
Another method according to the invention for controlling a paper machine, in which method the paper machine is controlled on the basis of data collected from it, is characterized in that the data is collected on at least one doctor blade forming part of the paper machine, which data is compared to previous data on doctor blades used in the same position and, at the same time, to data generally collected from the paper machine, so that if the collected data differ from the previous data the paper machine is controlled.
The apparatus according to the invention is applied in connection with a doctor blade and it is used to measure wear over the entire length of the doctor blade. The measurement can preferably be made continuously, so that the wear can be determined rapidly and accurately. The apparatus can be located in different positions, either permanent or movable. In addition, the apparatus is suitable for use in connection with different kinds of doctor blades. With the aid of the method according to the invention, the actual shape and extent of the wear can be determined. In addition, it is possible to influence the causes of the wear more rapidly than before.
These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings.